Current-limiting low-voltage fuses



June 7, 1960 F. J. KozAcKA 2,939,934

CURRENT-LIMITING Low-VOLTAGE FUsEs Filed Aug. 1a, 1958 United States Patent CURRENT-LIMITING LOW-VOLTAGE FUSES Frederick J. Koza'cka, South'Hampton, NH., assignor t0 The Chase-Shawmut Company, Newburyport, Mass., a corporation of Massachusetts Filed Aug. 18, 1958, Ser. No. 755,807

6 Claims. (Cl. 200-131) This invention relates to electric low-voltage fuses, and more particularly to such Afuses adapted to generate arc voltages which are high in comparison to the circuit voltage.

Fuses generating arc-voltages which are high in comparison to the circuit voltage are often referred to as highcapacity fuses, which term emphasizes the ability of such fuses to interrupt very high available currents. Fuses generating high arc-voltages are often designed to interrupt high fault currents before such currents can reach their `available peak values, and high-capacity fuses having this particular operating characteristic are generally referred-to as current-limiting fuses.

This invention relates to high-capacity ffuses, and more particularly to high-capacity fuses of the current-limiting type.

Fuses of the aforementioned character comprise a pulverulent arc quenching filler which has an intense cooling or deionizing action such as, for instance, a body of quartz sand. Such fuses further comprise a -fuse link of a high conductivity low -fusing energy metal (silver, copper) having a plurality of serially related points of reduced cross-sectional area. The arc-quenching action of the pulverulent ller and the geometry of the fuse link are so correlated as to result in a predetermined peak -arc voltage per inch of link length upon blowing of the yfuse by major fault currents. The peak arc voltage depends also upon the parameters of the circuit into which the =fuse is inserted, and in 'fuses of the kind under consideration will be in the order of many hundreds of volts per inch of link length. It is well known in the art how to determine the geometry and the length of a fuse link required to develop the desired arc voltage when submersed in an arc-quenching ller having a predetermined Iarc-quenching action. The length of a `fuse link `for a highcapacity fuse, and more particularly for a current-limiting fuse, when determined from the viewpoint of interrupting capacity requirements (and current-carrying capacity requirements) may be much shorter than the length of a fuse link of a Standard or National'Electrical Code fuse. This is due to the fact that a larger arc voltage per unit of link length may be generated in fuses of the aforementioned type than in `fuses of the last mentioned type, thus calling for a much shorter active link length in one case than in the other.

I-f the dimensions of the fuse link and the fuse tube of a current-limiting low voltage fuse are determined in accordance with interrupting capacity requirements, and inv accordance with current-carrying capacity requirements, then one may iind that the outer minimum distance trom the mid-point of the fuse to the nearest live part thereof tends to become too small. In the past many quite complex and expensive structures have been evolved to `free ultra-short high-capacity or ultra-short currentlimiting fuses `from the dangers resulting from too small a minimum distance from midpoint of the fuse to the nearest live part. To date no entirely satisfactory solution to the problem in hand has been offered.

Another school of thought considered the required safe minimum distance `from midpoint of fuse to nearest live part as the prime requirement around which any fuse 2,939,934 Patented June 7, 1960 ice whether of the Standard type, or of the high-interruptingcapacity type, or of the current-limiting type, ought to be designed. This school of thought provides any fuse, including current-limiting fuses, with a `fuse tube of standard length and diameter, and provides inside of such a tube ller and fuse link means designed to generate per inch of fuse tube length substantially smaller arc'voltages than those which could and would be normally generated in a current-limiting fuse of that size. Prior art fuses of this design have a fuse link which is substantially equal in length to that of the `fuse tube and is designed to generate considerably smaller arc-voltages per unit of length than the arc voltages generated in compact current-limiting fuses designed on the basis of required minimum dimensions ifor given interrupting capacity requirements. To provide current-limiting fuses with fuse links which are longer than dictated by interrupting capacity requirements tends to result in relatively high voltage drops across the terminals of the fuse, and relatively high fz'r losses inside the fuse, which both are serious drawbacks.

It is one object of this invention to provide novel and improved high interrupting capacity fuses, particularly novel and improved current-limiting fuses which have the advantages of both of the aforementioned types of prior art fuses, and which are not subject to the limitations or disadvantages of either of these types of fuses.

Another object of the invention is to provide improved high-interrupting capacity fuses, particularly improved current-limiting fuses, which have Vfuse tubes of standard sizes but smaller voltage drops and 2r losses than comparable prior art fuses.

Another object of the invention is to provide improved high-interrupting capacity fuses, particularly improved current-limiting fuses, which are designed for optimal interrupting eiiiciency and have the same minimum distances from mid-point to nearest live part as standard or National Electrical Code fuses.

The foregoing and other general and special objects of lthe invention and advantages thereof will more clearly appear from the ensuing particular description of the invention, as illustrated in the accompanying drawing, wherein:

Fig. l is a longitudinal section of a current-limiting fuse embodying the invention;

Fig. 2 is a side elevation of the struct-ure shown in Fig. l; and

Fig. 3 is a cross-section along 3 3 of Fig. l drawn on a larger scale than Fig. l.

The fuse according to this invention comprises a pulverulent arc-quenching rlller 1 having a very high heat absorbing capacity, preferably quartz sand. Three ribbon-type fuse links 2 of a metal combining the property of high electric conductivity with that of low fusing energy are submersed in the body of quartz sand 1. Each fuse link 2 has a plurality of points of reduced cross-section established by multiperforation of the link ribbon. Casing or fuse tube 2 accommodates the body of quartz sand 1 and the fuse links 2. The three fuse links 2 have a predetermined geometry and length. Their geometry and their length and the arc-quenching action of ller 1 are so correlated as to result in a predetermined average peak arc voltage per inch of link length upon blowing of the fuse by major fault currents. Assuming that the fuse has a current rating of 200 amps. and starts to limit available currents above 3,700 R.M.S. amps. and is able to interrupt available currents up to 100,680 R.M.S. amps. All other conditions remaining the same, the peak arc voltage tends to change with the rate of rise of current. It should never exceed 21/2 times the circuit voltage, and yet be suiciently high -to force the current from its let-through peak to zero before the time of the first natural current zero following fault in- 'e'ption The geometry or geometrical configuration links 2 and the arc-quenching capacity of iiller 1 are such as to result in dangerously high arc voltage peaks if j2, and V'l'rioe l'particularly the perforated portion eref, ad 'nner l1fxteitd'sussrantany. throughout .the

mirelleigthor fue@ tubecr casing 3. Filler 1 extends 'tally throughoutthe entire'length of the 'fuse tube 3, the length'of fuse links 2 is substantially less than e len'gfh of fusetube 3. The relative shortness of the niks :minimizes 'the voltage drop across the fuse and i i losses and heat generation in the fuse when carryits rated current and` relatively small overload lcur- The links 2, Vin spite'off their shortness, are able Yeiect a Vvcurrentlimiting interruption of the faulted ton inajorifaults. This is dueto the multibreak "fe ure-of links 2 resulting from being multiperforated, Icombined with the nature ofthe pulverulent arc-quench- 2ing filler 1 `rwith whichl links'2 are associated. Both ends lt 'h lrelatively long fuse tube`3 are closed by closing eme s in the Yform of fcaps "4, permanently secured to I"fusietube 3 by `pins v6, or by conventional means 'not 'fshoiinsuch as, for instance, crimping or riveting. The `relatively great length of the fuse tube 3 compared to ,tlatlof the fuse links 2 provides a safe minimum distance betvveen the midpoint or the fuse tube 3 and the nearest live part, namely caps or ferrules 4. The'fact that thel entire lengthof the casing or fuse tube 3 lis `filled with a potent deionizer such as quartz sand makes it possible to dispense `with expensive pressure-resistant terminal elements such as,`for instance, copper plugs, for zclosingthe fuse tube and to resort to simple inexpensive ing or terminal lelements in the 'form of sheet metal `-taps'il. The fuse further comprises a'pair of knife-blade contacts 5, or equivalent connectors, each projecting a ossonqeof caps 4'and secure-d to casing 3 by the pin 6 .ding caps 4'in1position. vEach blade contact or coniriectorS has one end projecting axially outwardly be-` Fs'elinks -2Aare preferably'imade of silver, but could also be maderof copper, if desired. Other metals hav* "irig A'1a substantially dierent conductivity and ya sub- 1, stantially different yfusing energy from silver and copper Willn't The distance betvveen the perforations in kstch'as to preclude merger by backburnies arclets incident to the occurrence of 'rajorfaultcurrents This design of fuse links has been fdesc'ribed nroreindotail in United States Patent 2,592,- 39`9f'nto WilliamlS. Edrsalh'et al., April 8, 1952, CurrentitingFuse, and reference may be had to this patent lfurther"'details'thereof` As also shown in the above sll'et lal. pateiit fuse links Z are provided with ineans interruption of the circuit on. the occurrence io g protracted overloads of relatively small intensity. To this fend'feach fuselink 2 is yproyided with a rivet 2. Vof

infor `ansuitable 'alloy4 of `tin adapted to destroyer inrrupt links "Ziby corrosion occurring above the fusing fp ointof tin. When'rivets 2 melt, the'silver vof links 2 Vi djacent to rivets 2' dissolves in the melted rivet metal and dilfsesiinto the same,`fthus forming tin-silver alloys of'frelyatively high resistivity, and reducing the cross- 'ioaljareasoflinks 2, until the same are eventually irely severed. v t, f V t 'Ilte'operationof the frise structure shown is innes- 4 nce`the`fsarne asthat of the structure disclosed inthe I@aboveEdsall et al.f patent. There are, however, some important distinctions. The `eilrrent-limiting fuses disclosed in this .patent are intended te` beA 1Qnsu'uctedv Vas compactas possible. Due to the increase of the ratio 'of the volume of the body of quartz sand to the interrupting capacity (Whichrnay be in the order of 100,000 amps.) inherent in the 'present structure, the transient pressiire incident toblowing =is 'greatly reduced, -and this B "ecasetuses embodying theipr'elsentinv'ention gen- A-er'jate muchhigherfircvoltages than'standard fuses the length of Vtheir links must besubstantially less than the standard:casinglengthffor the particular current'rating, v but since thefrequired'arc voltage isl `achieved without resortingrto'highvintefnalpressrs.insideof fuse tube or casing 3, Ythe -rentire' constructionof the fuse structure requires relatively little ruggfedness 'orY mechanicalstrength.

. The ,',fusef'struoture sh'vvn in,l the vdrawing has been tested, andffoiind to operatiesatisfactory both in the low ;currpenttinte'rrupting rangeV andin the high-current interrupting range'aslongfas applied to theprotectionoi;l A.C. circuits. Theyery Ysame structure lends itself also to the ,"protectionQofiDfC. kcircuits having high availabley fault currents if Llinks 'Zareprovi'ded with 4 plate structures or "are chutes ofthe kndvdisclosed inmy copending patent application Ser. No. 658,162Liiled May'9, 1957 for CurrentJimiting Fuses Increased Interrupting Capac- AYity, now United States Patent 2,866,038,'issued Eecember l It will be understood thatthbughr but one v,embodiment ,of the inventiony has been illustrated'and described in detail, the invention is notlimited thereto. It will also be "understoodthat the structurerillustrated may be modified ..Without departing fronjrthey spirit and scope of the inventionasfset forth in the accompanying claims.

, jI'claimV-as myinvention:

V1*.MIn an electric*current-limiting,low-voltage fuse the Ncombinationkof a body ofquartz sand; ,a ribbon fuse link ofl a metaly combining theproperty of relatively high con- Y ductivity lwith that of relatively low fusing energy, said i:usenlink Aforming a plurality of serially related points of reduced cross-section andrbeing submersed in said body minimum distancemfrorn midpoint ofthe fuse to lnearest ,life Part; a ,fuser'fube cfa sYnthetic-reSin-slass-cloth .lammate acconfundatinsv Saidbodyf, quartz sand and .Said

fuse link, the length of said fse tube considerably ex- Alhm ceeding said length of said fuse link and being at least tv ice said safe minimum distance from midpoint of the Euse to nearest life part; a pair of sheet metal caps closing both ends of said fuse tube; and a pair of knife-blade contacts arranged in substantially the same plane as said fuse link each projecting transversely across one of said pair of caps, each of said pair of knife-blade contacts having one end projecting axially outwardly beyond said fuse tube and each of said pair of knife-blade contacts having another end projecting axially inwardly into said body of quartz sand; and means providing conductive connections between the axially outer ends of said fuse link and the axially inner ends of said knife-blade contacts.

2. An electric fuse as speciiied in claim l wherein said fuse link is of silver and is spot-welded to said pair of knife-blade contacts.

3. In an electric current-limiting low-voltage fuse the combination of a pulverulent arc-quenching quartz filler; a multiperforated ribbon-fuse link made of a relatively high conductivity relatively low fusing energy metal and having a predetermined length submersed in said iiller; the arc-quenching action of said tller and the geometry of said fuse link being so correlated as to result in a predetermined average peak arc-voltage per inch of link length upon blowing of said fuse by major fault currents; a fuse tube of a synthetic-resinegiass-cloth laminate accommodating said ller and said fuse link, the length of said fuse tube being such as to result in dangerously high arc-voltage peaks if said filler and the perforated portions of said link were extending substantially throughout the entire lengthof said fuse tube, said filler extending substantially throughout the entire length of said fuse tube and the length of said fuse link being substantially less than the length of said fuse tube; a pair of closing elements permanently secured to said fuse tube at the axially outer ends thereof; a pair of connectors each projecting transversely across one of said pair of closing elements, each of said pair of connectors having one end projecting axially outwardly beyond said fuse tube and each of said pair of connectors having another end projecting axially inwardly into said arc-quenching ller; and fused means providing permanent conductive connections between the axially outer ends of said fuse link and the axially inner ends of said pair of connectors.

4. In an electric current-limting lowvoltage fuse the combination of a body of quartz sand; a fuse link of silver having a plurality of serially related points of reduced cross-sectional area submersed in said body of quartz sand; the arc-quenching action of said body of quartz sand and the geometry of said fuse link being so correlated as to result in a predetermined average peak arc voltage per inch of link length upon blowing of said fuse under major fault conditions; `a fuse tube of a synthetic-resinglass-cloth laminate accommodating said body of quartz sand and fuse link, the length of said fuse tube being such as to result in dangerously high arc voltage peaks if said body of quartz sand and said fuse link were allowed to extend substantially throughout the entire length of said fuse tube, said iiller extending substantially throughout the entire length of said fuse tube and the length of said link being substantially less than the length of said fuse tube; a pair of caps closing both ends of said fuse tube; a pair of knife-blade contacts each projecting transversely across one of said pair of caps, each of said pair of knife-blade contacts having one end projecting axially outwardly beyond said fuse tube and each of said pair of knife-blade contacts having another end projecting axially inwardly into said body of quartz sand; and means establishing fused conductive connections between the axially outer ends of said fuse link and the axially inner ends of said pair of knife-blade contacts.

5. In an electric current-limiting low-voltage fuse the combination of a pulverulent arc-quenching quartz iiller; a. fuse link of a high conductivity low fusing energy metal having a plurality of serially related points of reduced cross-sectional area submersed in said filler; a fuse tube accommodating said iiller and said fuse link and being substantialy equal to standard length as deiined by the table below:

the length of said fuse link being substantially less than standard length as deiined by the table above; a pair of terminal elements adapted to permanently close the ends of said fuse tube; a pair of connectors each projecting transversely across the general plane of one of said pair of terminal elements, each of said pair of connectors having one end projecting axialy outwardly beyond said fuse tube and each of said pair of connectors having another end projecting axially inwardly into said `arcquenching iiller inside said fuse tube; and means providing permanent conductive connections between the axially outer ends'of said fuse link and the axially inner ends of said pair of connectors.

6. In an electric current-limiting low-voltage fuse the combination of a body of quartz sand; a multiperforated ribbon `fuse link of silver submersed in said body of quartz sand; a fuse tube of a synthetic-resin-glass-cloth laminate `accommodating said body of quartz sand and said fuse link, the length of said fuse tube being substantialy equal to standard length as defined by the table below;

the length of said fuse link being substantially less than standard length as defined by the table above; a pair of terminal caps closing the ends of said fuse tube; a pair of knjfeblade contacts each projecting trnsversely cross one of said pair of terminal caps, each of said pair of knife-blade contacts having one end projecting axially outwardly beyond said fuse tube and each of said pair of knife-blade contacts having another end projecting axialy inwardly into said body of quartz sand inside said fuse tube; and means providing fused conductive connections between the axially outer en'ds of said fuse link and the axially inner ends of said pair of knifeblade contacts.

References Cited in the ille of this patent UNITED STATES PATENTS 1,016,524 Sachs Feb. 6, 1912 2,592,399 Edsall et al Apr. 8, 1952 2,625,626 Matthysse Ian. 13, 1953 2.866.875 Swain et al. Dec. 30, 1958 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTIUN Patent No. 2,939,934 June 1.960

Frederick J. Kozaoka It is hereby certified that error appears n the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read' as corrected below.

Column 6, lines 58 and 59, for "trnsversely cross" read transversely across Signed and sealed this lst day of November 11.960o

( SEAL) Attest: n*

KARL H. AXLINE ROBERT C. WATSON Commissioner of Patents Attesting Ofcer UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Fetent Noo 2,939,934 June T, 1.960

Frederick .I Kozaeka It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should readas corrected below.

Column 6, lines 58 and 59, for "transversely Cross" read transversely across Signed and sealed this lst day of November l960 Attest:

KARL H AXLINE Atesting @Hicer .ROBERT C. WATSN Commissioner of Patents 

